A substrate is an underlying solid material used in manufacturing products such as integrated electronic circuitry and microelectromechanical systems (MEMS). MEMS result from a technological advancement that unites silicon-based microelectronics with micromachining technology with the goal of producing complete systems on a single chip.
Integrated circuit and MEMS manufacturing comprise stepwise patterning and layering processes. Examples of such processes include the use of plasma to etch circuit-defining pathways, deposition of metals in the pathways to form circuitry, and application of chemicals and abrasives to etch, strip and/or and polish contact surfaces for high precision manufacturing. The processes begin with a suitable substrate, such as a wafer of crystalline silicon, upon which materials having the requisite electrical characteristics are deposited. Water and various chemicals may then be used to treat the surface of a substrate. The treatment can comprise cleaning, etching, or rinsing the substrate after each manufacturing step to quench reactions and ensure precision in the final product.
The process steps in the manufacture of integrated circuits offer many opportunities for contaminants to enter the structure of the product semiconductor substrate. Physical contamination is undesired matter and can comprise organic and inorganic materials such as particles, films from photoresist material, and traces of any other impurities such as metals deposited during implanting or etching. Semiconductor substrate cleaning may thus be the most frequent step in manufacturing integrated circuits and is becoming more critical as the features of semiconductor substrates get smaller. There are various methods of cleaning semiconductor wafers, and the process of choice must not only satisfy technical requirements, but must also satisfy environmental regulations and be cost effective.
The technical goal of cleaning a semiconductor substrate is to eliminate physical contamination between each process step without affecting the integrity and detail of the substrate provided by previous steps. Contamination of the surface of the substrate with undesired matter can affect the manufacturing process and reduce ultimate product performance. Thus, ways of avoiding contamination are paramount in the manufacture of electronic circuitry, as are ways of efficiently removing undesired matter without introducing further contaminants. Some cleaning methods developed to satisfy these goals have been discussed in the literature, for example, Int. Conf. On Solid State Devices and Materials, pp. 484-486 (1991); Kujime, T., et al., Proc. Of The 1996 Semi. Pure Water and Chemicals, pp.245-256; and, Singer, P. Semi. International, p. 88, (October 1995).
Patterning of integrated circuitry involves depositing material directly on a semiconductor substrate or intervening layers, and each patterning step typically involves the following: applying a photoresist to the surface of the substrate; changing the properties of selected areas of the photoresist by exposing those areas to light, X-rays, or particle beams such as electron or ion beams; removing either exposed or unexposed portions of the photoresist to expose portions of the underlying substrate; chemically treating or depositing material on the exposed portions of the substrate; and removing the residue. Each step in the patterning process can introduce a variety of contaminants, such as various residues, and must usually be followed by a cleaning step before proceeding to the next step in the process.
Etching generally refers to the removal of material from the surface of the semiconductor substrate and includes the pattern defining process. Each layer on the substrate is manufactured individually and then polished to obtain a precise match between layers. Currently, “wet etching” is used to etch semiconductor substrates in a chemical bath, whereas “dry etching” is used to define circuit pathways using a plasma. In dry etching, the plasma is used to form the circuit pathways and is commonly used because of the high precision and selectivity afforded by the process. However, the disadvantage to dry etching is the formation of post-etch residue (PER), which is a difficult to remove by-product of the reaction between the plasma, the substrate surface, and other material present such as the photoresist.
Post-etch residue is found around etched pathways and openings and may be comprised of ashed resist, etching gases, and etched substrate materials. Any post-etch residue must be removed to avoid reduced product performance due to interference from impurities in the intricate pathways or the formation of corrosive chemical species within the residue. One means of removing such contaminants is the use of organic solvents, but such solvents have required operating temperatures of as high as 100° C., often followed by a rinse with volatile and highly flammable solvents. Combining high temperatures with an easily ignitable rinse is clearly less than desirable. Although techniques that do not use isopropyl alcohol have been described, see for example, U.S. Pat. No. 5,571,337, they use vapors of other organic compounds.
Another process that utilizes cleaning chemistries is chemical mechanical polishing (CMP). CMP is a planarization process that combines wet etching with an abrasive slurry to remove excess material between layers in the semiconductor manufacturing process and is as crucial to high product performance as metal deposition or lithography. Planarization improves the contact between the wafer, the dielectric insulators, and the metal substrates, but also increases the room for error in other process steps. Given the onward march towards miniaturization, CMP is becoming a more and more critical step in the manufacturing process, but contaminants introduced during CMP must also be effectively removed.
Since the features of semiconductor wafers are now becoming as small as 0.10 microns, and dimensions of 0.07 microns are projected to occur by the year 2005, thorough removal of contaminants, whether present originally or introduced in preceding process steps, is becoming more critical than ever. Ideally, the sizes of particle contaminants should not exceed one tenth of the minimum feature size. Accordingly, cleaning procedures should thus be effective at removing particles as small as about 0.007 to about 0.010 microns. On these dimensions, the laws of physics produce unexpected results that are a function of the diminishing importance of mass (See e.g., Brown, D., “Surface Tension Rules the Subminiature World of MEMS,” available at http://www.engineer.ucla.edu/stories/mems.htm). In practice, in the submicron world, effects attributable to the inertia of particles are dwarfed by forces such as surface tension and adhesion. The critical forces acting on a submicron particle are those that are manifestations of electrostatic attraction and repulsion over ranges that are typically thought of as short in the macroscopic world but which are comparable to the size of the particles in the submicron regime.
At dimensions of 0.10 microns and less, most semiconductor substrates will need to use conductive materials with low dielectric constants (low-k materials), and such materials are inherently delicate. Low-k materials known in the art include: fluorinated silicate glass (FSG); hydrido organo siloxane polymer (HOSP); low organic siloxane polymer (LOSP); nanoporous silica (“Nanoglass”); hydrogen silsesquioxane (HSQ); methyl silsesquioxane (MSQ); divinysiloxane bis(benzocyclobutene) (BCB); silica low-k (SiLK); poly(arylene ether); (PAE, “Flare”, “Parylene”); and fluorinated polyimide (FPI). As a result, the emphasis in techniques such as CMP has become more “chemical” than “mechanical,” and there has even been a move towards abrasive free methods. It is also becoming more important to have CMP formulations that are not overly aggressive to delicate materials used with these intricate geometries due to the added problems such as erosion and delamination. Accordingly, a need exists for an effective CMP chemistry that will effectively remove small dimension contaminants without deleterious effects on manufacturing materials.
In most manufacturing processes, the substrate must not only be cleaned with a cleaning agent after each process step but must also be rinsed to remove residual cleaning agent before the next step. For example, an amine based cleaning agent can leave trace amounts of amine, which may be corrosive to metal substrates such as aluminum. Thus, a post-cleaning treatment is necessary to neutralize residual amines. Traditionally, an unreactive organic solvent may be used to dilute such reactants, and then a solvent of higher vapor pressure, such as isopropanol, is used to rinse away and dry the substrate. However, as previously mentioned, the flammability of such solvents is a disadvantage.
Preferred rinsing agents will selectively neutralize chemicals without reacting with other materials. An example of a commonly used rinsing chemistry is dilute NH4OH with dilute HF for post-CMP cleaning of tungsten wafers. Dilute HF is commonly used to remove the remaining monolayer amounts of organic or inorganic contaminants including metals and anions, but unlike organic chemistries, even dilute HF can damage the semiconductor substrate if not carefully controlled. Formulations that are safe and selective for post-cleaning and post-CMP rinsing are presented in U.S. Pat. Nos. 6,156,661 and 5,981,454 both of which are incorporated herein by reference.
In addition to neutralizing cleaning chemicals, it is also important to prevent redeposition of contaminants after cleaning. Isopropyl alcohol, deionized water, and ultrasonic or megasonic cleaning have traditionally been used in various combinations to remove particles, but other means of removal, both physical and other, have also been used.
One means of removal is megasonics, in which high pressure waves in a liquid solution push and tug at contaminants on a surface, effectively dislodging them. It has been found, however, that megasonics is only effective at removing particles as small 0.3 microns and is not expected to be effective at removing particles that are an order of magnitude smaller. Scrubbing and related techniques have been found to be an improvement upon megasonics.
An example of a physical means of removing particles is buoyancy. Buoyancy is illustrated in Japanese Patent No. 63-239982-A2 and U.S. Pat. No. 4,817,652, where it was shown that gas bubbles could lift dust particles away from the surface of a semiconductor substrate. Gas bubble formation in liquid solution was induced around dust particles, and the buoyancy of the gas bubble released and lifted the particle from a substrate to the surface of the solution. Surface tension forces were described as part of the particle removal mechanism in that the film encasing the bubble would rapidly converge underneath the particle and detach the particle from the surface of the substrate. Thus, a buoyant force is used to overcome an adhesive force. If the surface tension between the liquid and the substrate is higher than that between the liquid and the particle, the liquid will prefer to remain attached to the substrate. Consequently, the liquid will prefer to pass between the particle and the substrate rather than just pass over the particle.
A further example of a physical means of removal is based upon the use of differences in interfacial surface tension. In U.S. Pat No. 4,781,764, an advancing and retracting “interface of a liquid” was taught as a method of detaching particles from the surface of substrates that were too small to be effectively removed using megasonics. The important surface tension relationship is the difference between two values: the interfacial surface tension between the liquid and the substrate and the interfacial surface tension between the liquid and the undesired matter. The movement of the liquid film over a surface creates a force on that surface, and the amount of force created depends on the interfacial surface tension between the liquid and the surface. As such, differences in interfacial surface tensions between the undesired matter and the semiconductor substrate assist in removing particles by “scrubbing” undesired matter from the semiconductor substrate. This physical means of removal was found to be an improvement over the use of megasonics in the removal of smaller particles.
Thus, since some residues are more effectively removed through chemical techniques, while others are more effectively removed by interfacial scrubbing, there is a need for a cleaning technique that is effective at removing a variety of substances at the scales required for the dimensions of the features on current and future semiconductor wafers. Such a technique must also be capable of being used efficiently in an industrial environment and a variety of formulations.
A foam is an agglomeration of gas bubbles separated from one another by a thin liquid film. In U.S. Pat. Nos. 6,090,217 and 6,296,715 B1, both of which are incorporated herein by reference, a foam was taught as useful for drying, cleaning and chemically treating a substrate. Cleaning chemicals such as ammonium hydroxide, hydrofluoric acid, hydrogen peroxide and nitric acid were reported, though all of these have known corrosive effects on delicate substrates and patterns deposited on substrate surfaces. However, foam compositions utilizing non-aqueous solvents in combination with cleaning chemicals were not disclosed. In particular, foam formulations that included corrosion inhibitors or chelating agents were not disclosed. Furthermore, foam techniques for removal of post-etch residue, or for carrying out CMP, were not taught.
A preferred method of foam formation, as described in U.S. Pat. Nos. 6,090,217 and 6,296,715, was the introduction of carbon dioxide gas into a liquid solution, accompanied by appropriate controlled variations of pressure to create a foam. Although carbon dioxide has a surface-tension reducing effect on an aqueous solution, at higher concentrations it produces an acidic solution and may not be compatible with other cleaning reagents. Other methods of facilitating foam production involved the addition to a liquid formulation of surface-tension reducing agents such as surfactants. A foam that could remain stable for approximately one to two minutes could deliver cleaning chemical to the semiconductor substrate using about one tenth of the amount of liquid and chemical normally required to achieve the necessary concentration, thus achieving a cost saving.
It was envisaged that the foam bubbles individually wetted the substrate surface, thereby forming a continuous film of liquid over the substrate surface that replicated the action of an equivalent liquid formulation but at considerably less cost. During foam application, the foam flowed over the substrate, and eventually discharged into an overflow container before decaying and draining. A disadvantage of using foam was that the foam must remain stable and in contact with the substrate long enough to deliver cleaning chemical. It was also envisaged that foam action was attributable, at least in part, to a “scrubbing” effect in which the substrate moves relative to the foam and the mass of foam bubbles dislodges particles from the surface.
Nevertheless, although an advantage of foam compositions and processes is that less liquid and chemical is necessary to achieve the same amount of cleaning as that achieved using liquid phase semiconductor cleaning, etching, and rinsing technology, formulating effective foam chemistries is difficult. Unpredictable criteria such as effective means of foam production and stability militate against universal applicability of foam techniques, however. A further principal disadvantage of current foam technology is that it doesn't provide methods and foam compositions for chemicals that are capable of cleaning post-etch residue.